Shares of Occidental Petroleum Corp. /zigman2/quotes/207018272/composite OXY +4.27% rallied 3.6% in afternoon trading Tuesday, enough to pace the S&P 500's /zigman2/quotes/210599714/realtime SPX -0.86% energy sector gainers, after the oil and natural gas company disclosed that Warren Buffett's Berkshire Hathaway Inc. /zigman2/quotes/200060694/composite BRK.B -0.99% boosted its stake in Occidental to more than 20%. Occidental disclosed in a late-Monday filing that Berkshire Hathaway increased its investment in Occidental by 6.68 million shares, valued at $390.72 million, to 188.37 million shares, or 20.2% of the shares outstanding. At current prices, Berkshire Hathaway's common stock stake is valued at $11.72 billion. The shares were purchased in a series of transactions from Aug. 4 through Aug. 8, at a weighted average price of $58.4763, according to a MarketWatch analysis of filing data. That price is 6.0% below current levels. The purchase further distances Berkshire Hathaway as Occidental's largest shareholder, as the second-largest shareholder is Dodge & Cox at 10.75%, according to FactSet data. Occidental's stock has soared 114.6% year to date, while the SPDR Energy Select Sector ETF /zigman2/quotes/206420077/composite XLE +1.83% has climbed 34.3% and the S&P 500 /zigman2/quotes/210599714/realtime SPX -0.86% has shed 13.5%.